Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. More particularly, the process of microlithography, in conjunction with the process of etching, is commonly used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is typically first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is usually exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist can be developed to produce an image corresponding to the mask pattern. Then an etch process can be used to transfer the pattern into the thin film stacks on the wafer. Finally, the photoresist is usually removed. Repetition of this process with different masks can result in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of an elongated rectangular slit, for example.
There are two types of commonly used projection exposure apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction while synchronously scanning the wafer stage parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection objective, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning device. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, for example masks realized as a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. Also programmable LCD arrays may be used as active masks. For the sake of simplicity, the remainder of the disclosure is provided in the context of apparatus that include a mask and a mask stage, but the general principles discussed in such apparatus are to be understood in the broader context of the patterning devices noted above.
In some cases, it can be desirable for the illumination system to illuminate each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions along which the rays constituting the light bundle propagate. The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known for modifying the angular distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In a relatively simple case a stop (diaphragm) which includes one or more apertures is positioned in a pupil surface of the illumination system. Because locations in a pupil surface translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the pupil surface help determine the angular distributions in the mask plane. However, in general, any change of the illumination setting involves a replacement of the stop. This can make it difficult to finally adjust the illumination setting, because this could involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations.
Many common illumination systems include adjustable elements that make it possible, at least to a certain extent, to continuously vary the illumination of the pupil surface. Conventionally, a zoom axicon system including a zoom objective and a pair of axicon elements are used for this purpose. An axicon element is a refractive lens that has a conical surface on one side and is usually plane on the opposite side. By providing a pair of such elements, one having a convex conical surface and the other a complementary concave conical surface, it can be possible to radially shift light energy. The shift can be a function of the distance between the axicon elements. The zoom objective can help make it possible to alter the size of the illuminated area in the pupil surface.